Para-xylylene polymers are employed as coatings for various electronic components due to their desirable physical and electrical properties. One advantage of polypara-xylylene coatings is that thin layers of such coatings are capable of exhibiting highly desirable physical and electrical properties. Because para-xylylene coatings are applied in very thin layers, heat tends to dissipate rapidly from the underlying components. Thus, the coated components cool down quickly and are less prone to temperature related degradation than similar components bearing other types of coatings.
In further contrast to conventional polymer coatings, para-xylylenes are generally not prepolymerized prior to application on the coatable substrates. This is because the para-xylylene polymers are not given to simple extrusion, melting or molding as are many of the conventional thermoplastics. Additionally, because the para-xylylenes are generally insoluble in commonly used organic solvents, it is impractical to employ traditional solvent deposition techniques for applying poly-paraxylylene coatings.
Accordingly, in most commercial applications, para-xylylene polymers are deposited on desired substrates by a pyrolytic deposition process known specifically as the "parylene process." Such process begins with the vaporization of a cyclic di-para-xylylene dimer. The dimer is pyrolytically cleaved at temperatures of about 400 to 750 degrees C. to form a reactive para-xylylene monomer vapor. Thereafter, the reactive monomer vapor is transferred to a deposition chamber wherein the desired substrates are located. Within the deposition chamber, the reactive monomer vapor condenses upon the desired substrates to form a para-xylylene polymer or co-polymer film.
Any monomer vapor which fails to condense within the deposition chamber is subsequently removed by a cold trap which is maintained at approximately minus 70 degrees C.
The entire parylene process is generally carried out in a closed system under constant negative pressure. Such closed system may incorporate separate chambers for the (a) vaporization, (b) pyrolysis, and (c) deposition steps of the process, with such chambers being connected by way of appropriate plumbing or tubular connections.
A primary consideration in the parylene deposition process is the achievement of uniform coating thickness on the desired substrates. Unlike conventional polymer coating systems, the condensation deposition of parylene coatings is capable of depositing even ultra-thin films without running or uneven areas resulting upon the substrates, provided that the monomer vapor is homogeneously and evenly distributed on the surface of the substrate. Thus, the design and functioning of the deposition chamber is critical to the achievement of uniform vapor distribution with resultant even coating deposition. Another important consideration in the parylene deposition process is the minimization of waste. Because of the high costs associated with parylene raw materials, there exists substantial economic motivation to preserve and conserve the parylene materials during the coating process. One particular. area in which a great deal of material is wasted is on the various internal structures of many prior art parylene deposition chambers. It must be appreciated that the condensation deposition of coatings is not substrate selective, i.e. the vapors have no way of seeking out only the desired substrates. Instead, the monomer vapor will condense and polymerize on any reduced temperature object with which it comes in contact. As a result, the entire inner surface of the chamber, and all of the objects positioned therein will become covered with the parylene coating. Thus, the interior of the chamber and any existing hardware must be cleaned periodically to remove the wasted parylene polymer.
The parylene deposition chambers employed in the prior art have generally provided less than optimal coating uniformity due to the inferior distribution and homogeneity of the vapor within the deposition chamber. Also, because of the particular chamber design, the prior art deposition chambers are associated with a great deal of waste of the parylene chemicals. Though many of the aforementioned deficiencies in prior art deposition chambers have been alleviated by the device disclosed in U.S. Pat. No. 4,945,856, issued to the subject applicant, this particular device in and of itself possesses certain deficiencies which detract from its widespread use.
Foremost of these deficiencies is the relatively high cost and large size of the parylene deposition chamber and the system with which it is utilized. Particularly, the system disclosed in U.S. Pat. No. 4,945,856 is modular in configuration and comprises an independent parylene deposition unit which is interfaced to and detachable from a pyrolytic generating unit. Though the modular construction of the device allows the units to be separated and allows the single pyrolytic vapor generating unit to be used with multiple interchangeable deposition chambers, the size of the combined units still makes set up of the system difficult, particularly in those environments where limited floor space if available. The present invention overcomes this particularly deficiency as well as the coating deficiencies associated with the prior art devices by providing a deposition chamber which is positionable upon a table top and is adapted to obtain uniform coating thickness while minimizing parylene polymer waste.